Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications, such as angina pectoris and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory Parkinson's Disease, and DBS has also recently been applied in additional areas, such as essential tremor and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neurostimulation systems typically includes one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. Thus, electrical pulses can be delivered from the neurostimulator to the stimulation electrode(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. The best stimulus parameter set will typically be one that delivers stimulation energy to the volume of tissue that must be stimulated in order to provide the therapeutic benefit (e.g., pain relief), while minimizing the volume of non-target tissue that is stimulated. A typical stimulation parameter set may include the electrodes that are sourcing (anodes) or returning (cathodes) the stimulation pulses at any given time, as well as the magnitude, duration, and rate of the stimulation pulses. The neurostimulation system may further comprise a handheld patient programmer to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The handheld programmer may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer Station (CPS), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
Typically, the volume of activated tissue in any given neurostimulation application may be increased or decreased by adjusting certain stimulation parameters, such as amplitude and pulse width. However, the size of the volume of activated tissue is not modified in a continuous fashion, but rather in a discrete fashion, where the step sizes of the increasing or decreasing stimulation energy are constrained by the amplitude and pulse width resolutions permitted by the hardware used to generate the stimulation energy. Insufficient resolution is a problem in applications where tissue associated with the therapy and the tissue associated with undesirable side effects are juxtaposed, such as in DBS or SCS. That is, given the current resolution of the stimulation hardware, it may be difficult to stimulate the target tissue that provides the therapeutic relief without also stimulating the tissue that causes the side effects.
Evidence exists that the current hardware resolution used to increase or decrease the volume of activation in DBS with existing Food and Drug Administration (FDA) approved devices is too large. For example, in subthalamic nucleus (STN) stimulation, clinicians typically use pulse widths close to the short end of the available range (60 μs minimum)(See The Deep-Brain Stimulation for Parkinson's Disease Study Group, Deep-Brain Stimulation of the Subthalamic Nucleus or the Pars Interna of the Globus Pallidus in Parkinson's Disease, N Engl J Med, Vol. 345, No. 13, Sep. 27, 2001), and one candidate possibility for the use of short pulse widths is that they allow smaller changes in the volume of activation for a given amplitude step size (e.g., the Kinetra® IPG allows 0.05V steps, and the Precision® IPG allows 100 μA steps) than do large pulse widths. See Gorman and Mortimer, The Effect of Stimulus Parameters on the Recruitment Characteristics of Direct Nerve Stimulation, IEEE Transactions on Biomedical Engineering, Vol. BME-30, No. 7, July 1983.
FIG. 1 simplistically illustrates a problem that may result from having a stimulation resolution that is too low to adequately stimulate target tissue T without stimulating non-target tissue NT. As there shown, stimulation energy is applied to an electrode E at two increasing amplitudes (A1, A2, and A3) to create three increasing volumes of activated tissue (V1, V2, and V3). The stimulation energy applied to the electrode E at amplitude A2, although not stimulating the non-target tissue, is inadequate to include the entire target tissue within the volume of activated tissue V2, thereby failing to optimize the therapy provided to the patient. The stimulation applied to the electrode E at amplitude A3, while sufficient to include the entire target tissue T within the volume of activated tissue V3, also includes the non-target tissue NT, thereby potentially creating undesirable side effects.
There, thus, remains a need for a neurostimulation method and system modifying a volume of activation with an increased resolution.